The invention relates to a distance measuring circuit which may be used in a photographic camera or the like.
Currently, a distance measuring circuit which is utilized in an auto-focus circuit of a photographic camera operates on the principle of a double image coincidence technique in which the image of an object being photographed is projected onto a pair of light receivers, and the image projected onto one of light receivers is mirror-scanned under the resilience of a spring in response to the depression of a release button in order to determine the point where the images projected on both of the receivers coincide with each other to thereby determine the distance to the object being photographed. However, the mirror scan under the resilience of a spring prevents a plurality of distance measuring operations to be performed in succession. In addition, if the release button is rapidly depressed, the sequence may not proceed in a proper form.
As an alternative, a distance measuring circuit is proposed which utilizes the attenuation of the light intensity in proportion to the square of the distance. Specifically, a light beam is continuously projected to an object being photographed by means of a light projector, and reflected light from the object is received by a light receiver in order to detect the output signal level of the receiver. However, the detection of the intensity of reflected light from the object being photographed causes a reduced accuracy in the determination of the distance as a result of variations in the light output from the projector. The accuracy is further reduced if extraneous light is incident on the light receiver or electrical noises are applied to the output signal therefrom.
In the latter arrangement, the distance may be subdivided into three ranges, for example, short distance, medium distance and long distance, and a determination may be made as to in which range the object is located. In such instance, the signal from the light receiver is amplified by an amplifier before it is compared against a pair of different reference voltages ref1 and ref2 in a pair of comparators, which determine in which range the object is located. In this instance, it is necessary to provide an arrangement such that an input voltage to the comparators do not saturate intermediate the reference voltages ref1 and ref2, as illustrated in FIG. 1, where Vs represents a saturation point. In the event the amplifier has a low saturation point or utilizes a low supply voltage, the reference voltages ref1, ref2 will be located close to each other, increasing the likelihood that a wrong determination may be caused by noises.
In the latter arrangement, a Schmidt circuit is used to eliminate noises from an output signal from the light receiver. While the use of such Schmidt circuit allows noises as produced by fluorescent lamps or the like to be eliminated, a hysteresis in operating level is introduced to degrade the accuracy.